In the past, when cutting a workpiece such as a steel plate with a laser processing apparatus, the workpiece is laser-pierced to form a small through-hole with a diameter of several millimeters at a point on the workpiece serving as a cutting start point, and the cutting starts from the through-hole.
The through-hole is important in improving finishing precision of the workpiece without decreasing the yield of material, and is preferably formed to a desired size, as small as possible, but not too small to perform a subsequent cutting operation.
FIG. 17 illustrates a schematic structure of a laser torch 400 used in a laser processing apparatus. The laser torch 400 includes a nozzle 402 and a condenser lens 404. The nozzle 402 has a cylindrical shape, and is configured such that a laser beam L2 can pass through a proximal end 402a of the nozzle 402 toward an opening 402b provided at a distal end thereof. The condenser lens 404 is provided at the proximal end 402a. 
The opening 402b of the nozzle 402 is coaxial with the laser beam L2 having passed through the condenser lens 404.
The nozzle 402 is provided with an inlet port 403 for introducing an assist gas G into the nozzle 402 when melting and evaporating the workpiece W by irradiation of the laser beam L2; the assist gas G reacts with the workpiece W, causing oxidation and burning of the workpiece W during the melting and evaporating operation.
When processing a piercing hole H2 on the workpiece W using the laser torch 400, the opening 402b of the nozzle 402 is opposed to the workpiece W, the assist gas G is introduced from the inlet port 403 into the inside of the nozzle 402, and the introduced assist gas G is injected from the opening 402b of the nozzle 402 so as to cover a processing portion of the workpiece W.
Next, a laser beam L1 is irradiated from the laser torch 400 and is then condensed by the condenser lens 400, providing a laser beam L2 having a focal point proximal to the surface of the workpiece W. In this way, the laser beam L2 focused at a focal point proximal to the surface of the workpiece W melts and evaporates the workpiece W to form a molten pond 405. Meanwhile, the molten material in the molten pond 45 is removed by the flow of the assist gas G by being oxidized and burnt by the assist gas G injected from the opening 402b. For example, Patent Document 1 describes a technology for processing the piercing hole H2 using a laser beam.
In the processing of the piercing hole H2, the excessive oxidation and burning of molten metal in the molten pond 405 of the workpiece W increases the diameter of the piercing hole H2, decreasing the yield of material.
Meanwhile, when processing the piercing hole using a pulsed oscillation laser beam, it is possible to improve the precision of the piercing hole diameter, but the processing efficiency may deteriorate greatly.
For example, Patent Document 2 describes a technology for improving the cutting precision (hole precision) of the piercing hole H2 when the piercing hole H2 is processed by the use of a laser beam.
However, the technology described in Patent Document 2 needs to provide a discharge groove for discharging the molten material in advance, and therefore, the size of the piercing hole is increased by the presence of the discharge groove.
That is, in the piercing processing, the excessive oxidation and burning of the workpiece W increases the piercing hole diameter, deteriorating the yield of material. Meanwhile, the piercing processing employing a pulsed oscillation laser beam can improve the precision of the piercing hole diameter but greatly deteriorates the processing efficiency.
For this reason, there is a desire for a laser piercing method and a processing apparatus enabling high precision processing of a piercing hole to a desired size while providing high processing efficiency.    [Patent Document 1] JP-A-2001-47268    [Patent Document 2] JP-B-3292021